The present invention relates to the field of gas analysis downhole in a hydrocarbon producing well. More particularly, the present invention relates to a system for analyzing carbon dioxide concentration downhole in a borehole
Hydrocarbon producing wells contain numerous formation liquids and gases including methane, ethane, carbon dioxide, hydrogen sulfide, and other gaseous compounds. Deep wells produce fluids at extremely high temperatures. The detection and analysis of gas concentrations provides information useful in evaluating the commercial value of a hydrocarbon producing well. Numerous systems have been developed to evaluate borehole fluid composition and the relative gas concentrations in the borehole fluid.
In U.S. Pat. Nos. 5,167,149 to Mullins et al. (1992) and in U.S. Pat. No. 5,201,220 to Mullins et al. (1993), a borehole apparatus detected the presence of gas within a formation fluid sample. A light source transmitted light to an interface between the fluid sample and a flow line, and a detector array sensed reflected light rays having angles of incidences between the Brewster angle and the critical gas angle. A processor determined the percentage of gas by comparing the detected information to information stored in a data base. The processor also categorized the fluid sample as high gas, medium gas, and low gas.
A method for determining the quantity of dissolved gas in a sample was disclosed n U.S. Pat. No. 5,635,631 to Yesudas et al. (1997), wherein the pressure and volume of a sample were first measured. The sample pressure was changed by expanding he sample until the pressure/volume relationship was non-linear, and the sample was expanded to determine the point at which pressure was unchanged. A bubble point for the sample was determined, and the sample pressure and the bubble point volume was determined. The dissolved gas volume was then calculated by linearly scaling the bubble point volume and the extrapolated sample volume relative to the difference between the second volume and the bubble point volume.
Other systems have been disclosed to evaluate gas composition within a formation fluid. U.S. Pat. No. 4,994,671 to Safinya et al. (1991) disclosed a borehole logging tool for analyzing the composition of formation fluids. Specifically, the apparatus used near infrared spectral analysis to determine quantities of gas, water and oils in a hydrocarbon fluid. A light source emitted near infrared rays in a wavelength range between 0.3 and 2.5 microns, and a spectral detector sensed the spectrum of backscattered and transmitted rays. A data base stored the sensed data, and a processor determined the fluid composition by evaluating the near infrared absorption spectral information. The source spectrum and either the transmitted or backscattered light spectra were compared to known spectral data. After the bubble point or dew point was identified, the low line pressure was increased above such point by controlling the fluid flow rate or by moving the logging tool to an appropriate depth within the borehole.
In certain boreholes containing formation fluids combining various liquids and gases, carbon dioxide occupies substantial volumes relative to the amount of recoverable hydrocarbons. As the formation fluids are produced to the wellbore surface, hydrocarbon gases are separated from the noncommercial carbon dioxide because the economic value of carbon dioxide relative to the hydrocarbons is low. Although carbon dioxide has been historically discharged into the ambient surroundings, global warming issues may discourage this form of gas separation and disposal. Carbon dioxide is also reinjected into subsurface geologic formations, however re-injection wells are expensive and may be impractical in certain geographic regions.
Accordingly, a need exists for a system capable of accurately evaluating the presence and quantity of carbon dioxide downhole in a borehole so that zones containing high levels of carbon dioxide can be avoided. The system should accurately identify the carbon dioxide concentration under different pressures, temperatures and wellbore conditions, and should provide real-time logging capabilities before borehole completions operations are performed.